The manifestation of strainbursts is related to the strain energy stored in the rock and how this energy is released and transitioned during unstable dynamic failure. It is of significance to ...investigate the role of energy evolution and transition during strainbursts and how it is influenced by surrounding rocks. For this purpose, experimental and numerical studies were performed in this study. Composite rock–coal specimens were loaded under the uniaxial compressive condition to produce strainbursts in the laboratory. Numerical simulation was then performed to reproduce the strainburst process observed in the laboratory and investigate the energy-absorbing-transition process associated with the spontaneous instability of the coal in the composite coal–rock specimens. The result has provided direct evidence of the conversion of elastic energy stored within a rock into the forms of kinetic energy to be released during the strainbursts process. For composite rock specimens under unconfined compressive conditions, a softer rock with lower Young’s modulus tends to store more elastic energy than a stronger rock and thus contribute more to the strainbursts that occurred at a surrounding rock. This loading condition is analog to the pillar loading condition in which the pillar is loaded by the roof and floor of the pillar due to the stress distribution resulting from excavation. The kinetic energy of ejected rock in strainbursts is not solely from the elastic energy stored in the zone where the bursts occur but also from the surrounding rocks. Most of the kinetic energy comes from the elastic energy stored within the busting zone and the contribution of the elastic energy stored in surrounding rocks is not significant.
Most strokes are covert and observed incidentally on brain scans, but their presence increases risk of overt stroke and dementia. Amyloid angiopathy, associated with Alzheimer Disease (AD) causes ...stroke, and when even small strokes coexist with AD, they lower the threshold for dementia. Diffuse ischemic white matter disease impairs executive functioning, information processing speed, and gait. Neuroimaging techniques, such as tissue segmentation, Diffusion Tensor Imaging, MR Spectroscopy, functional MRI and amyloid PET, probe microstructural integrity, molecular biology, and activation patterns, providing new insights into brain-behavior relationships. MR-pathological studies of periventricular hyperintensity (leukoaraiosis) in aging and dementia reveal arteriolar tortuosity, reduced vessel density, and occlusive venous collagenosis which causes venous insufficiency and vasogenic edema. Activated microglia, oligodendroglial apoptosis, clasmatodendritic astrocytosis, and upregulated hypoxia-markers are seen on immunohistochemistry. Further research is needed to understand and treat this chronic subcortical vascular disease, which is epidemic in our aging population.
Deep underground coal mines face great challenges of extreme squeezing conditions resulting from high mining-induced stresses caused by a high recovery of the reserves and low to moderate strength ...coal measures. This is particularly true for deep roadways that suffer side and front abutment pressures. In this study, a case study was conducted in a 1000-m deep coal mine with extreme squeezing ground conditions. A new strategy of “ground support + rock modification + destressing” was proposed for the ground support. A numerical simulation was performed to evaluate the performance of the new support strategy. Field observations suggested that the proposed strategy could successfully mitigate the extreme squeezing of the double-used entry. The proposed support strategy provides an effective measure for ground control in extreme squeezing conditions.
A distinct element grain-based method (GBM) was developed to simulate the microstructure of rock-like materials. Using this method, a UDEC-GBM model can be readily constructed with a given mineral ...composition, allowing independent assignment of specific properties to both the grains and grain boundaries. Both intra-granular cracks cutting through the grains and inter-granular cracks developed along grain boundaries can be captured. These features allow a full incorporation of both geometric and mechanical heterogeneity at grain scale for simulating brittle rocks. The validity of the proposed UDEC-GBM approach was verified by simulating a low-porosity sandstone under compression and direct-shear tests. The UDEC-GBM was proved to be capable of reproducing many of the characteristics associated with brittle fracture in low-porosity sandstone. It was found that the model with unbreakable grains trends to under-estimate the crick initiation threshold, highlighting the importance of the incorporation of breakable grains when modeling micro-structure of brittle rocks. The numerical experiments suggested that examining the extent of the tensile stress zones alone may lead to a biased evaluation of tensile cracking at crack initiation. The tensile stress magnitude must also be taken into consideration. It was also found that a synthetic sandstone sample with relatively low ground boundary strength produces a more ductile post-peak behavior. Microscopic tensile strength of the grains has limited influence on the failure mechanism of the synthetic specimen under unconfined compression loading. The proposed GBM approach provides a very useful tool for studying grain-scale micro-mechanics of brittle rocks.
One of the most challenging safety problems in underground longwall coal mining is massive roof collapse in mined-out area during retreat mining. Understanding the mechanisms and geotechnical ...precursors of massive roof collapse is of great importance for risk assessment and ground control. In this study, a large-scale physical model was created based on a real case to simulate massive roof collapse during longwall coal retreating mining. A good agreement was achieved between field observation and physical result in terms of monitored working resistance, indicating that the physical model is able to capture the realistic response of the rock strata above mined-out area. A numerical model was created based on the model configuration of the physical model. The physical result and the numerical result were in good agreement with each other in terms of many features including roof failure mechanism, abutment pressure distribution and collapsed roof pattern. The physical and numerical result suggested that massive roof collapse is a typical snap-through failure where bedded roof crack through cross joint at midspan. Substantial increase in horizontal stress at the center of the potential failure zone is a reliable geotechnical precursor of massive roof collapse. It is suggested that horizontal stress increasing up to five times pre-mining can be considered as a threshold of the precursor of massive roof collapse.
•Massive roof collapse mechanism was evaluated through physical and numerical modeling.•Massive roof collapse is a typical snap-through failure where bedded roof crack through cross joint at midspan.•Substantial increase in horizontal stress at the center of the potential failure zone is a reliable geotechnical precursor of massive roof collapse.
This paper reviews the major achievements in terms of mechanical behaviors of coal measures, mining stress distribution characteristics and ground control in China's deep underground coal mining. The ...three main aspects of this review are coal measure mechanics, mining disturbance mechanics, and rock support mechanics. Previous studies related to these three topics are reviewed, including the geomechanical properties of coal measures, distribution and evolution characteristics of mining-induced stresses, evolution characteristics of mining-induced structures, and principles and technologies of ground control in both deep roadways and longwall faces. A discussion is made to explain the structural and mechanical properties of coal measures in China's deep coal mining practices, the types and distribution characteristics of in situ stresses in underground coal mines, and the distribution of mining-induced stress that forms under different geological and engineering conditions. The theory of pre-tensioned rock bolting has been proved to be suitable for ground control of deep underground coal roadways. The use of combined ground control technology (e.g. ground support, rock mass modification, and destressing) has been demonstrated to be an effective measure for rock control of deep roadways. The developed hydraulic shields for 1000 m deep ultra-long working face can effectively improve the stability of surrounding rocks and mining efficiency in the longwall face. The ground control challenges in deep underground coal mines in China are discussed, and further research is recommended in terms of theory and technology for ground control in deep roadways and longwall faces.
Rockbursts cause damage to underground excavations in a sudden and violent manner and are associated with mining-induced seismic events. We propose a simple experimental method to study strainburst ...process in the laboratory, that simply involves a common compression testing apparatus and rock-coal-rock specimens. Strainbursts of coal samples are successfully produced and the burst process is monitored using high-speed camera and acoustic emission sensors. The strainburst mechanism is characterized by an initial ejection of small coal fragments followed by the ejection of large coal blocks. We found that the strainbursts are caused by the elastic strain energy stored in the rock samples during the uniaxial compression. The amount of the transferred energy is significantly less than the elastic energy stored in the coal sample but plays an important role in triggering strainbursts. The greater the transferred energy, the greater the damage severity of strainbursts occurred in the coal sample. Tensile cracking subparallel to the vertical loading direction and tangential compressive stress appears to play a dominant role in the strainburst failure mechanism.
In underground mining practice, the rock bolt support system is the major support pattern to control the deformation and stability of openings. A rock bolt is generally subjected to complex loads ...including tension, torsion, bending and shear, which result from the deformation of excavations and exposure to dynamic loads that are generated by rockbursts. An understanding of the response of rock bolt under complex conditions is of great importance for rock bolt support design and practice. New sophisticated equipment has been developed for this purpose. This work involved a comprehensive experimental study on the mechanical behavior of rock bolts under complex loads. The results show that rock bolt pre-tensioning by torque application to the nut can result in decreases in tensile strength and elongation because the rock bolt is subjected to a combination of tension and distortion. When a pre-tensioned rock bolt is subjected to a shear load, the maximum shear force can reach up to 80% of the tensile capacity of the rock bolt. Higher impact energy results in a longer period of dynamic loading and a larger irreversible plastic deformation on the rock bolt, in contrast to a rock bolt that is subjected to low impact energy. The capacity and especially the deformation capacity of a rock bolt may decrease significantly after successive containment of the deformation of the surrounding rock mass from rockbursts.
In underground coal mining, coal rib failure of longwall entries is almost certain due to the relative weakness of coal and the presence of high mining-induced stresses. To maintain the coal’s ...capability of sustaining large loads after failure and substantial deformation, it is crucial to understand its post-peak behavior and subsequently, its residual strength. In this study, triaxial compression tests were performed on a total of 51 coal specimens that were classified into two groups based on the existence of calcite grains. Particular emphasis was given to evaluating the residual strength of the coal specimens. It was found that the presence of calcite grains has significant effect on the brittleness of the coal. Coal specimens containing calcite grains have lower frictional strength than specimens without calcite grains. The tests demonstrated that splitting is completely suppressed when the confinement exceeds 10 % of the coal’s unconfined compressive strength. The Coulomb and Hoek–Brown failure criteria satisfactorily fit the coals’ the peak strength data over the entire confining stress range. The Hoek–Brown failure criterion satisfactorily fits the residual strength data. The parameter
m
for residual strength is significantly greater than that of the peak strength for both groups of coal. It was also found that as the confinement increases, the increase in residual strength is greater than the increase in peak strength.
•Fault-slip induced seismic waves are captured using a proposed DEM method.•Both P- and S- waves can trigger rockbursts, depending on its vabriting direction.•Seismic waves can be successfully ...blocked by a soft zone with low Young’s modulus.•The proposed method shows promise for simulating earthquakes.
Numerical modeling of fault-slip rockbursts remains a great challenge due to the inherent difficulties in the simulation of fault slippage and rock failure under dynamic ground conditions. A sophisticated numerical method based on the distinct element method is proposed to simulate fault-slip rockbursts. The method successfully simulates the generation of a seismic event induced by fault slip. The seismic event generates seismic waves that spread out in a pattern that vibrating in a direction parallel to the fault and moving in the fault normal direction then triggers rockbursts when reaching an opening. Many features associated with seismic waves are produced using the proposed method including the fault growth process, the radiation pattern of the P- and S- waves, and the dissipation of seismic energy. The effect of fault geometry on the characteristic of the seismic event and the triggering of rockbursts is examined. It is found that the magnitude of the seismic event generated by fault slippage is related to the stress field of the fault. Both P and S waves can trigger rockbursts, depending on whether the wave is incident on the tunnel surface at an oblique angle. The seismic waves generated by fault sliding can be successfully blocked by a soft zone. The findings of this study provide useful insight into the source mechanism of rockbursts and the control of the accompanying damage. The proposed method shows promise for simulating earthquakes.
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